The present invention relates to systems for determining the position of mobile stations relative to base stations, and particularly to such systems which employ pseudorandom noise code signals emitted from known base stations and received by the mobile stations.
Various positioning systems are known, such as, for example, radio navigation systems in which timed signals at differing frequencies are transmitted from a plurality of base stattions and received by mobile stations. The received signals are compared to a locally generated reference signal on each mobile station and a range determination is made relative to each base station. These systems include Loran-C, Raydist, Argo, Shoran and Transit.
Loran-C is a pulsed 100 KHz technique that takes advantage of the low propagation loss at low frequencies but suffers from range inaccuracies of 200 feet or more at 500 miles.
Raydist is a continuous wave phase measurement technique at 2000 KHz that also takes advantage of low propagation loss but with increased distance accuracy due to a higher carrier frequency relative to Loran-C. Over the horizon accuracies of a few meters are practical. This system suffers from overwhelming skywaves at long distances and at night-day transitions causing loss of ranging data without the ability to reacquire range when the signal returns.
Argo is a pulsed, phase comparison system at high frequencies having similar skywave propagation problems to Raydist.
Shoran, as well as Maxiran, and Trisponder are UHF Pulse Systems that provide 5 to 15 feet range accuracy over line of sight pathss, 50 to 75 feet range accuracy over the horizon to 150 miles in polar latitudes and occasionally 300 miles in tropical latitudes. The long range usefulness of these systems suffers from intermittent operation being dependent upon the presence of atmospheric ducts for its signals at ranges beyond 60 to 100 miles.
Transit is a Navy navigational satellite system which offers unlimited range but whose position fixes are intermittent due to the flyby nature of the satellite and therefore not useable for continuous precision positioning.
The continuing search for offshore oil sources has been pushing further offshore each year. This search requires precision navigation to allow accurate dynamic exploration measurements such as seismic surveys, bottom profiling and relocation of previously drilled wellheads at long distances from reference points. An accurate navigation system for this type of application must be capable of making range measurements out to at least 400 miles, handle boat speeds of up to 10 to 20 knots, and be repeatable in range tolerances of + or -20 feet. It is also desirable that such a system be capable of operation 24 hours per day, have no range ambiguities within 400 miles, have data link capability for integration with other systems, and be capable of simultaneous operation with several users. None of the known systems are able to fulfill all of these criteria.
Presently, a high accuracy global position system (GPS) is being developed which uses pseudorandom type signals generated by a plurality of satellites for deriving positioning information on the earth. U.S. Pat. No. 4,114,155 to Raab shows an example of a receiver used to interpret the pseudorandom satellite generated signals. However, the obtainable accuracy from a GPS system is marginal with respect to current requirements, and, as a military system, may not be available for general use.
U.S. Pat. No. 3,714,573 to Grossman shows a spread spectrum system capable of monitoring the positions of vehicles. In the Grossman system, each vehicle contains apparatus which repetively transmitts a uniquely coded spread spectrum identification signal asynchronous to the repetitive identification signals transmitted by all other system vehicles. However, the Grossman system only allows the position of each vehicle to be monitored at a central location; no provision is made for the vehicles to be able to monitor their own positions as is required in offshore navigation.